Sir Menzies Campbell will reassert his leader's authority over the Liberal Democrat conference when he attacks the "comfortable, cosy and complacent consensus" between Labour and the Tories - and tells his activists that only they can break it. In an exclusive interview with the Guardian last night Sir Menzies rejected fears that his new strategy risks voters seeing the Lib Dems as too leftwing - and declared himself a lifelong man of the "centre-left".

"I'm a politician in the centre-left, I joined a centre-left party, I'm leading a centre-left party, I make no secret of that," he said. "But it's others that have moved, not us, and I think the real divide is between not so much left and right but from liberal and authoritarian. And we are the anti-authoritarian party."

Party officials later revealed that he will also use today's speech to deride "part-time liberals" in a jibe against the Tory leader, David Cameron, whose claims to be a "liberal conservative" have faded under pressure from his own rightwing. Closing his party's conference in Brighton, Sir Menzies will accuse his main party rivals of contesting a narrow scrap of political ground on the centre-right. It leaves the Lib Dems as the only party "prepared to take the lead and speak the truth" on a raft of issues, including the environment, taxation, tuition fees and the Iraq war.

Officials still insist the party is ready for Gordon Brown to go to the polls as early as October, despite criticisms of Sir Menzies' leadership which yesterday's boost in the Guardian's ICM poll - the Lib Dems were 2% up to 20% - have not quelled.

Sir Menzies has done 60 interviews in Brighton and last night called the week "fine and fun". His spokesman promised he would tackle head-on question marks over his leadership and the lurking battle for succession. Yesterday's agenda was coloured by jockeying between Nick Clegg, the party's home affairs spokesman, and Chris Huhne, the environment spokesman.

Mr Clegg's admission that he would probably challenge for the leadership, though only once Sir Menzies stood down, prompted criticism that he was premature from Mr Huhne, his likely rival.

Sir Menzies again insisted that "there is no vacancy. I shall lead this party through this parliament, through the general election, and into the parliament beyond" and said he was totally relaxed about remarks by "young Turks" he had been delighted to promote. In his interview last night he warned would-be contenders: "When the moment comes, it won't just be Huhne and Clegg, there will be others, including at least one woman. There's some very bright cookies around."

But his true feelings may have been betrayed by his wife, Lady Elspeth. As she passed Mr Clegg in the Grand Hotel she said: "I don't know if you're being helpful or not." In an exchange captured by ITN, Mr Clegg replied: "I'm trying to be." At her side Sir Menzies said: "He's being very helpful." Lady Campbell lamented: "It's so difficult."

Mr Clegg justified his remarks at an Observer fringe meeting: "I'm afraid I'm not the kind of person who dances on the head of a pin and ducks questions. If someone says if there's a vacancy some time in the future, might you be in the frame, I'm not going to be playing this disingenuous game of ruling anything in or out."

Mr Clegg's support coalesces strongest around a group of young, rightwing modernisers and Mr Huhne has played well to the traditional liberal wing, though both men resist such caricatures. Both were criticised by Simon Hughes, the party president. "I am extremely concerned about this ridiculously early start for the next internal election for the leadership of the party. It really is not fair to the current incumbent that up and coming candidates are positioning themselves so early...unlike Tony Blair, when Ming said he was elected for a full term he meant it."